1. Field of the Invention
This invention relates generally to patterned perpendicular magnetic recording media, such as disks for use in magnetic recording hard disk drives, and more particularly to an improved method for making patterned disks by nanoimprint lithography or similar patterning techniques.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In conventional continuous magnetic recording media, the magnetic recording layer is a continuous layer over the entire surface of the disk. In patterned media, also called bit-patterned media (BPM), the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. While BPM disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer, will likely be the choice for BPM because of the increased data density potential of perpendicular media. To produce the magnetic isolation of the patterned data islands, the magnetic moment of the spaces between the islands are destroyed or substantially reduced to render these spaces essentially nonmagnetic. Alternatively, the media may be fabricated so that there is no magnetic material in the spaces between the islands. The spaces between the disks may then be filled with nonmagnetic material and the disk may then be planarized to provide a smooth surface.
One technique for making BPM is by etching a full film through a lithographically-patterned mask. Nanoimprint lithography (NIL) is one type of lithographic technique that has been proposed. NIL is based on deforming an imprint resist layer by a master template or mold having the desired nano-scale pattern. The master template is made by a high-resolution lithography tool, such as an electron-beam tool. In one technique, the structure to be imprinted is the disk substrate with the magnetic recording layer and any required underlayers formed on it as continuous layers. Then the liquid imprint resist is deposited on the recording layer. In a modified technique, a hard mask layer is formed on the recording layer and the resist is deposited on the hard mask layer. The imprint resist may be a polymer curable by ultraviolet (UV) light, such as MonoMat available from Molecular Imprints, Inc. The transparent master template is pressed onto the liquid resist and then the resist is exposed to UV light to harden the resist. The template is removed from the imprint resist, leaving an inverse nano-scale pattern of recesses and spaces on the imprint resist. In a modified technique, a hard mask layer is formed on the recording layer and the resist is spin-coated on the hard mask layer. As an alternative to a UV-curable imprint resist, a thermoplastic polymer, like poly-methylmethacrylate (PMMA), may be used as the imprint resist. The polymer is heated above its glass transition temperature. At that temperature, the thermoplastic resist becomes viscous and the nano-scale pattern is reproduced on the imprint resist by imprinting from the template at a relatively high pressure. The patterned imprint resist layer is then used as an etch mask to form the desired pattern of islands in the underlying recording layer, or to transfer the desired pattern into the hard mask layer, which is then used as mask for an additional etching step to form the desired pattern of islands in the underlying recording layer.
The islands in BPM need to have sufficient magnetic quality, for example high coercivity (Hc) and saturation magnetization (Ms). The islands in BPM also need to be sufficiently small to support high bit areal densities (e.g., 1 Terabit/in2 and beyond). For example, data islands with diameters approximately 20 nm or less may be required. However, it is important that as the size of the islands decreases, the thermal stability of the islands is maintained. The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic recording material and V is the volume of the magnetic grain. Thus it is important that the islands maintain a high KuV for thermal stability. The BPM fabrication process can introduce a variety of defects in the magnetic islands, which suppress thermal stability and add undesirable variation to the island properties.
What is needed is a method for making a BPM disk with nanoimprint lithography that results in small data islands with sufficient magnetic quality and thermal stability.